Apparatus, system, and method for bus link width optimization

ABSTRACT

A bus permits the number of active serial data lanes of a data link to be re-negotiated in response to changes in bus bandwidth requirements. In one embodiment, one of the bus interfaces triggers a re-negotiation of link width and places a constraint on link width during the re-negotiation.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is generally related to bus power optimization for a bus comprised of a plurality of serial data lanes. More particularly, the present invention is directed towards optimizing bus power in peripheral component interfaces.

BACKGROUND OF THE INVENTION

There is an increasing need to reduce the power consumption of personal computers and other types of computing devices. A high-power consumption is undesirable because it increases the cooling requirements of the personal computer. Additionally, a high-power consumption is undesirable because it reduces the battery lifetime of mobile computers.

There are several components of a personal computer that draw significant amounts of power. First, the chip containing the central processing unit (CPU) draws power. Second, other chips, such as a graphics processing unit (GPU) or a bridge chipset, also consume power.

Additionally, a high-performance personal computer requires high-bandwidth data buses to transfer data between components. High-bandwidth data buses also consume a significant amount of power. The power required to operate a high-bandwidth data bus can be significant, particularly in the context of a mobile personal computer having a limited power budget.

Therefore, what is desired is an apparatus, system, and method for reducing bus power for a high bandwidth data bus.

SUMMARY OF THE INVENTION

An apparatus, system, and method is disclosed in which a data bus has a plurality of operable data lanes that may be associated to form a data link between components. The data bus permits the number of active data lanes of the data link to be adaptively adjusted in response to changes in bus bandwidth requirements. The bus is configured to have a sufficient number of active lanes to provide a high bandwidth for operational states requiring high bandwidth. However, for operational states requiring less bandwidth, the bus is configured to have a smaller number of active lanes sufficient to supply the reduced bandwidth requirement of the operational state, reducing the bus power requirements.

In one embodiment of a method of the present invention, in a first operational state having a first link bandwidth requirement, a first link width is negotiated having a first integer number, M, of active data lanes, where M is less than or equal to a maximum integer number, K, of operable data lanes. In response to detecting a second operational state having a second link bandwidth requirement, a second link width is negotiated having a second integer number, N, of active data lanes, where N is not equal to M, and N is also less than or equal to K.

In one embodiment of a bus of the present invention, the bus includes a first data interface, a second data interface, and a plurality of data lanes for communicating data between the first data interface and the second data interface. The first and second data interfaces are adapted to initiate a negotiation for a number of active data lanes corresponding to a subset of operable data lanes. The first and second data interfaces also re-negotiate the number of active data lanes to place a constraint on the number of active data lanes associated to form the link in response to a re-negotiation signal.

In one embodiment of a computing system of the present invention comprises a root complex, a CPU coupled to the root complex for executing a software application stored in a memory, an endpoint device associating a graphical processing unit, and a bus having a plurality of data lanes coupling the root complex to the endpoint device. The bus is configured to associate data lanes into a common link having a link width corresponding to a number of active data lanes selected to adapt to the bandwidth requirements of the graphical processing unit.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a system including a bus apparatus in accordance with one embodiment of the present invention;

FIG. 2 is a flowchart illustrating a method for negotiating bus bandwidth in accordance with one embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for negotiating bus bandwidth in accordance with one embodiment of the present invention;

FIG. 4 is a state diagram illustrating a method for triggering re-negotiation of bus bandwidth in accordance with one embodiment of the present invention;

FIG. 5 is a state diagram illustrating a method of triggering re-negotiation of bus bandwidth in accordance with one embodiment of the present invention;

FIG. 6 is a sub-state diagram of a detect state in accordance with one embodiment of the present invention;

FIG. 7 is a sub-state diagram of a polling state in accordance with one embodiment of the present invention;

FIG. 8 is a sub-state diagram of a configuration state in accordance with one embodiment of the present invention;

FIG. 9 is a sub-state diagram of a recovery state in accordance with one embodiment of the present invention;

FIG. 10 is sub-state diagram of a loopback state in accordance with one embodiment of the present invention;

FIGS. 11A and 11B are a flow chart illustrating a method of bus link width reduction in accordance with one embodiment of the present invention;

FIGS. 12A and 12B are a flow chart illustrating a method of bus link width increase in accordance with one embodiment of the present invention;

FIGS. 13A and 13B are a flow chart illustrating a method of bus link width increase in accordance with one embodiment of the present invention; and

FIG. 14 is a flow chart illustrating a method of adapting bus bandwidth for a graphics system in which a software application may run software applications having different processing requirements and different bus bandwidth requirements.

Like reference numerals refer to corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed towards an apparatus, system, and method for adaptively adjusting bus bandwidth. FIG. 1 is an illustrative block diagram of a system 100 in accordance with one embodiment of the present invention. Referring to FIG. 1, system 100 includes two components 105 and 110 that are communicatively coupled by a data bus 115 comprised of a plurality of serial data lanes 120 and bus interfaces 130 and 135.

In some embodiments, bus 115 is part of a system in which a first component 105 is an end-point device, such as a graphical processing unit (GPU). The other component 110 may correspond to a bridge, such as a root complex for peripheral devices. A CPU 170 and memory 180 having executable software instructions 190 may be coupled to component 110. However, it will be understood in the following description that bus 115 may be utilized in a variety of systems other than that illustrated in FIG. 1.

In one embodiment, each of the serial data lanes 120 corresponds to a pair of serial simplex data links to transmit and receive data. For example, each serial simplex data link may correspond to a high-speed differential pair with a transmitter disposed in one bus interface and a corresponding receiver in the other bus interface, with the high-speed differential pair having a data rate and associated data bandwidth. Each serial simplex data link may transmit an embedded data clock. A logical idle may be transmitted on the data lanes even when no data is being transmitted.

Each bus interface 130 and 135 includes its own local link state machine 140 for initializing a common data link between the components in which one or more of the data lanes are associated to form a common data link. With N active serial data lanes, where N is an integer, the link between the components is a “by N link” having N active lanes. The bandwidth of the link scales with the number of active lanes that are associated to form the link between the components. In particular, a transmission code, such as 8 bit/10 bit encoding, may be used to send serial data symbols on each data lane that represent bytes of a larger word of data transmitted on the bus, i.e., the word of data is encoded for transmission on one or more lanes of the bus.

Assuming that each data lane transmits data at the same rate, the bandwidth of the bus scales with the number of active lanes. Consequently, a by N link has a bandwidth proportional to its link width, where the link width is the integer number of active data lanes that are associated to form the common data link between the components. (The link width is also commonly referred to as the “bus width.”) Additionally, since only the active lanes (i.e., powered lanes with embedded clocks) consume significant power, the power consumption of the bus also scales proportionally with the number of active lanes used in the link.

At least one of the bus interfaces includes an adaptive link width controller 150. Adaptive link width controller 150 acts to adjust the integer number of active lanes in accordance with changes in bandwidth requirements for the bus, i.e., it selects a subset of operable data lanes to be used to transmit/receive data, where the subset is in the range of 1 to K lanes, where K is the maximum integer number of operable data lanes supported by the interfaces. This permits the link width to be adapted to be a minimum number sufficient to provide the required bandwidth, thereby reducing the power consumption of the bus during time periods when a low bandwidth is sufficient while permitting the bus bandwidth to be increased during time periods when a high bandwidth is required.

Referring to the flowchart of FIG. 2, in one embodiment the bus determines 205 operable serial data lanes and operating parameters for the data lanes. As an illustrative example, there may be a maximum integer number, K, of operable data lanes that are supported by both interfaces, where K is ≧2. At a certain point of time, the interfaces of the bus negotiate 210 an integer number, M (M≦K), of active data lanes for an operational linkwidth. The bus may then enter a normal operational state in which data may be exchanged across the bus using the M active data lanes. In response to a condition for which the bus bandwidth requirements change, the interfaces of the bus re-negotiate 215 the number of active data lanes used in the link to a different integer number, N, corresponding to a different link width (i.e., M≠N, N≦K). The bus may then enter a normal operational state in which data is exchanged across the bus using the N data lanes. The process may continue based on other conditions. For example, another condition may occur which triggers the interfaces of the bus to renegotiate 220 the number of active data lanes used in the link to a third integer number, L, of active data lanes.

For example, a computing system may have bus bandwidth requirements corresponding to two or more levels of bus traffic, such as low and high bandwidth requirements or high, medium, and low bandwidth requirements. The bandwidth requirements may be associated with the type of software being executed by system 100. For example, word processing applications commonly have comparatively low bandwidth requirements in comparison to high-performance three-dimensional graphics applications. The bandwidth requirements of a software application may be detected from an attribute of the software. For example, some software applications, such as high-performance graphics applications, include function or resource calls that are indicative of their bandwidth requirements. Additionally, some system states may be detected by hardware, such as idle states or busy “wait states” in which a portion of system 100 may be internally processing information such that the system generates comparatively little traffic for bus 115. Moreover, in one embodiment, power management software may reduce bus bandwidth in a low power mode and increase bus bandwidth for an active mode.

In one embodiment, adaptive link width controller 150 includes link width control registers 160 that may be written into by hardware or software detection elements (not shown in FIG. 1) when an event is detected corresponding to a change in the bandwidth requirements for bus 115.

In one embodiment, the bus interfaces 130 and 135 form the link between the components through a training sequence in which training sequence signals are sent back and forth between the bus interfaces to communicate information and coordinate the states of each interface. Among other tasks, the training sequence messages permit one of the interfaces to trigger the other interface to enter a different state of its link state machine 140. Thus, in a first training sequence a first link width may be negotiated whereas a second link width may be subsequently negotiated using a second training sequence.

Referring to the flowchart of FIG. 3, in one embodiment a first bus interface receives 305 a signal enabling re-negotiation of link width and which places a constraint on the bus link width. The first bus interfaces exits the normal operational state (sometimes known as “L0”) and generates 310 one or more training sequences selected to trigger itself and the second bus interface to enter a configuration state. In the configuration state, the first bus interface imposes 315 a constraint on the number of operable data lanes that may be used. This may, for example, include creating training sequence messages that indicate to the other interface that certain lanes are not to be used, even though they are operable lanes (e.g., by sending a signal indicating that one or more lanes are disabled). The bus completes configuration 320 and returns to the normal operational state with the modified bus lane width.

It is preferable to have one interface control the renegotiation process, i.e., both trigger the initial re-negotiation and also place a constraint on the lane width during the negotiation. However, it will be understood that in alternative embodiments one interface may trigger the re-negotiation process while the other interface places the constraint on the lane width during the re-negotiation.

Each re-negotiation process is significantly faster that the initial negotiation because one or more steps used in the initial link initialization process may be omitted in the renegotiation process. For example, a conventional bus reset or power up operation includes resetting the state of registers, detecting operable data lanes, and setting common communication parameters of the lanes. This corresponds to Reset, Detect, and Polling states used in an initialization process. The Reset, Detect, and Polling states require a substantial amount of time in comparison to the configuration state used to negotiate link width. Consequently, in the present invention each re-negotiation can be fast in comparison to a reset process.

FIG. 4 is a state diagram for link state machine 140 in one embodiment of the present invention. Conventionally, the link is initialized by triggering a reset state 405. A detect state 410 is entered to detect operable lanes. Subsequent to the Detect state, a Polling state 415 is entered to poll the lanes and arrive at common operating parameters such that the lanes may communicate training sequences. Subsequent to the polling state, a configuration state 420 is entered to configure the link. The configuration state 420 includes link width negotiation. After the link is configured, the bus enters an operational state 425 in which the bus is capable of transmitting data. In some embodiments, additional low power operational modes (e.g., L1 and L2 sleep states for the entire link width) may be entered from operational state 425. An example of an implementation of low power operational modes is described in the co-pending U.S. patent application entitled, “Method and Apparatus for Power Management of Electrical Components,” U.S. patent application Ser. No. 10/431,989, filed May 7, 2003, the same date as the present application and assigned to the same assignee as the present application, the contents of which are hereby incorporated by reference.

Referring to arrows 480 and 485, the renegotiation process uses one or more additional states of the link state machine to access the configuration state 420 from operational state 425. A recovery state 430 is sometimes included in a link state machine to permit a bus to recover an attribute of the bus. In one embodiment, the renegotiation process begins in the normal operational state 425. One of the interfaces generates a training sequence that causes both interfaces to enter recovery state 430. From recovery state 430 the interfaces then enter configuration state 420 and re-negotiate link width using any constraints on link width imposed by the initiating user interface.

FIG. 5 illustrates a state machine diagram for an alternate embodiment. Referring to arrows 480, 490, and 495, in one embodiment the initiating interface generates training messages that cause a transition to the Recovery state 430. From the recovery state 430 the state machines enter a Loopback state 435. Loopback states are sometimes included in bus interfaces to perform diagnostics of individual components. In this embodiment of the present invention, the Detect state 410 is entered from the Loopback state 435. Thus, the Reset state 405 is avoided. Moreover, as described below in more detail, in some embodiments, the interactions of the state machines may be adapted to eliminate some of the sub-states of the detect state.

The apparatus, system, and method of the present invention may be adapted for use in a variety of bus types and bus protocols. In one embodiment, bus 115 is compliant with the Peripheral Component Interconnect (PCI) Express™ standard and a PCI express implementation will now be described. PCI Express™ is a standard developed by the PCI-SIG industry organization. The protocols of the PCI Express™ specification are described in the “PCI Express™ Base Specification,” version 1(a), available from the PCI-SIG organization via the World Wide Web at the URL address: www.pcisig.com, the contents of which are hereby incorporated by reference.

In the PCI Express™ specification each point-to-point link between components may have 1, 2, 4, 8, 12, 16, or 32 dual simplex 2.5 Gbps lanes. PCI Express™ calls for 8 bit/10 bit data encoding to create a 2 differential pair, 2.5 Gbps (2.0 Gbps effective rate) dual simplex data stream with an embedded clock. In PCI Express™, a “symbol” is the 10 bit quantity produced as a result of 8b/10b encoding. The PCI Express™ specification calls for the link width to be negotiated to the maximum link width supported by both sides of the bus.

In a PCI Express™ implementation, link state machine 140 executes the states of the link training and status state machine (LTSSM) of the PCI Express™ specification. The PCI Express™ specification specifies mandatory LTSSM states and training set protocols. However, the PCI Express™ specification also permits individual developers to implement optional modifications as long as they are compatible with mandatory states and mandatory protocols. In the PCI Express™ protocol, training sequences are commonly denoted as TS1 or TS2 training sets.

In a PCI Express™ implementation the term “downstream” refers to a relative position that is farther away from a root complex and a downstream request is one in which information is flowing away from root complex (e.g., towards an endpoint device). Conversely, in a PCI Express™ implementation the term “upstream” refers to a relative position that is closer to the root complex and upstream request is one in which information is flowing towards the root complex (e.g., from an endpoint device).

Referring back to FIG. 1, in one embodiment of a PCI Express™ implementation the signal that triggers a link width renegotiation may be received by a re-negotiation control register(s) within the link width control registers 160. For example, a signal from hardware or software indicative of a change in bus bandwidth requirements may be used to generate an enable signal and maximum lane width written into re-negotiation control registers (RNCTRL). The maximum lane width estimate should be compliant with a supported integer number of lane widths. In one implementation, re-negotiation control register includes an enable bit, RNCTRL.Enable to enable re-negotiation and a maximum width bus field RNCTRL.Maxwidth to indicate the maximum number of lanes required.

In PCI Express™, training sequences are in the form of ordered sets TS1 and TS2 for initializing bit alignment, symbol alignment, and to negotiate physical layer parameters for operation, such as the link identification number and the lane ordering within the link. Control bits are included in TS1 and TS2 to perform specific functions. The TS1 and TS2 training sequences are used to discover and determine link width, link data rate, lane reversal, and polarity inversion. Training also includes link data rate negotiation, bit lock per lane, lane polarity, symbol lock per lane, lane ordering within a link, link width negotiation, and lane-to-lane deskew. In general, the PCI Express™ protocol uses TS1 training ordered-sets to provide proposals or counter-proposals of certain physical layer operational parameters and uses TS2 training ordered-sets to accept the proposals and confirm the parameters.

PCI Express™ also includes special symbols that are distinct from data symbols. These include special symbols used for link management. The symbol “PAD” is used in framing and link width and lane ordering negotiations.

Referring again to FIGS. 4 and 5, the LTSSM of each interface of a PCI Express™ implementation includes a Reset state 405, Detect state 410, Polling state 415, Configuration state 420, Normal Operational state 425, Recovery state 430, and Loopback state 435. Details of these states specific to practicing a PCI Express™ embodiment will now be described.

FIG. 6 is a block diagram illustrating a detect sub-state machine for implementing a detect state of a PCI Express™ LTSSM. In a detect state, the transmitter performs a receiver detection sequence on unconfigured lanes that can form one or more links. A Detect.Quiet state 605 is entered if a receiver is not detected on any lanes. If a receiver is detected on all active lanes, the Detect.Active state 610 is entered. If the Detect.Active state 610 succeeds in detecting receivers on all active lanes then the detect state exits to polling 615. A re-association process is initiated in the LTSSM for unconfigured lanes that did not detect a receiver. The PCI Express™ specification defines a protocol of training sets for each detect sub-state.

FIG. 7 is a block diagram illustrating a polling sub-state machine for implementing a polling state of a PCI Express™ LTSSM. In the polling state training ordered sets are sent and received. In the polling state bit lock is established, symbol lock is established, lane polarity is established, and lane data rate is established. The polling sub-states correspond to a Polling.Active state 705 that sends out TS1 ordered sets to indicate that polling is active and to establish bit and symbol lock, a Polling.Compliance state 710 for use with test equipment to assess if the bus is compliant with voltage and timing specifications, a Polling.Configuration state 715 sending out TS2 training ordered sets to confirm bit lock, symbol lock, and data rate, and to invert the polarity, if necessary, and a Polling.Speed state 720 to change the data rate to the highest data rate supported on both sides of the bus. The PCI Express™ specification defines a protocol of training sets for each polling sub-state.

FIG. 8 is a block diagram illustrating a configuration sub-state machine for implementing a configuration state of a PCI Express™ implementation. In the configuration state the lanes are configured into a link through a link width and lane ordering negotiation sequence. Referring to FIG. 8, the PCI Express™ specification includes the configuration sub-states of Configuration.Linkwidth.start 805 to initiate a link width negotiation by proposing a link identification number on the desired active lanes of the link, Configuration.Linkwidth.Accept 810 to accept the link identification number and to counter-propose the number of desired active lanes, Configuration.Lanenum.Wait 815 for proposing and counter-proposing the desired lane ordering, Configuration.Lanenum.Accept 820 for accepting lane ordering assignments, Configuration.Complete 825 to confirm completion of configuration, and Configuration.Idle 830 to place the link in a logically idle condition prior to the normal operational state. The PCI Express™ specification defines a protocol of training sets for each configuration sub-state.

FIG. 9 is a block diagram illustrating a sub-state machine for implementing a recovery state of a PCI Express™ LTSSM. The recovery state allows a configured link to re-establish bit lock, symbol lock, and lane-to-lane de-skew. In the recovery state the transmitter and receivers are sending and receiving data using the configured link and lane number as well as the previously negotiated data rate. The recovery sub-state machine includes a Recovery.Rcvrlock state 905 that sends ordered training sets to re-establish bit and symbol lock, a Recovery.RcvrCfg state 910 to re-establish lane-to-lane deskew and re-confirm the lane ordering assignment, and a Recovery.Idle state 915 that permits access to other states, based upon directions or other conditions. The PCI Express™ specification defines a protocol of training sets for each recovery sub-state.

FIG. 10 is a block diagram illustrating the sub-states of loopback specified in PCI Express™ for the LTSSM. Loopback state 450 is conventionally included to allow for component testing and fault isolation. During loopback data that is sent out is looped back to test data paths of the bus. These include a Loopback.Entry state 1005 to enter loopback, a Loopback.Active state 1010 for an active loopback mode, and a Loopback.Exit state 1015 for a condition exiting loopback. The PCI Express™ specification defines a protocol of training sets for each loopback sub-state.

In a PCI Express™ implementation, the two LTSSMs of the bus interfaces interact through TS1 and TS2 ordered training sets that are exchanged between the LTSSMs. The interactions between the LTSSMs cause them to move through a sequence of sub-states. In the present invention, at least one of the LTSSMs is adapted to create training sequences that initiate the link width renegotiation process and that places a constraint on the link width negotiation. As described below in more detail, in one embodiment the PAD symbol is inserted in training sequences to disable selected lanes.

FIGS. 11A and 11B are a flow chart illustrating a PCI Express™ implementation in which an end point device initiates the renegotiation process by triggering entry into the recovery state to enter the configuration state to renegotiate lane widths. Steps 1105–1140 illustrate a reduction of link width whereas steps 1145–1170 outline the steps for an increase in link width (which are illustrated in FIGS. 12A–13B in more detail).

Referring to FIGS. 11A and 11B, in one embodiment a hardware or software entity activates re-negotiation in the endpoint device. This includes asserting 1105 the RNCTRL.Enable bit and writing the RNCTRL.Maxwidth field of the renegotiation control register(s).

The interface compares 1110 RNCTRL.Maxwidth to the value of the LinkStat.Linkwidth, the currently negotiated bus width recorded in a link status register. If LinkState.Linkwidth, M, is greater than RNCTRL.Maxwidth, N, (e.g., N<M), it corresponds to a determination that the bus width can be reduced to save power. The endpoint device LTSSM then enters 1115 the recovery state. It sends out training sets TS1 and TS2 according to the current link and lane numbers as specified by the PCI Express™ for the Recovery.Rcvrlock and Recovery.Rcvrcfg states of the recovery sub-state machine.

As a result the upstream device (e.g., the root complex) is driven 1120 into the recovery state upon detecting the TS1/TS2 training sets sent by the downstream device. The link number and lane number fields in the TS1/TS2 remain the same as for previously negotiated values during Recovery.Rcvrlock and Recovery.Rcvrcfg. Since the link number and lane number fields are unchanged, the Recover.RcvrLock and Recovery.RcvrCfg sub-states may be completed comparatively quickly, e.g., around 32 TS1/TS2 periods, or 512 Symbol times. This results in both ends of the link being driven into the Recovery.Idle sub-state at the end of recovery.

At the recovery.idle sub-state, instead of placing the link into a locally idle state by sending null symbols, the endpoint device sends out 1125 TS1 ordered sets on all configured lanes with the link number the same as the original value. The lane number field of the TS1 ordered sets is assigned the value of “PAD”. The endpoint device enters the configuration sub-state Configuration.Linkwidth.Start.

The upstream component is driven 1130 into the configuration state by receiving TS1 ordered sets with lane number being “PAD”. In one embodiment, two consecutive TS1 ordered sets with lane number field being “PAD” on the Recovery.Idle state causes the upstream component to enter the Configuration.Linkwidth.Start sub-state.

With both components in the Configuration.Linkwidt.Start sub-state, link width re-negotiation begins. During re-negotiation, the endpoint device uses the RNCTRL.MAXWIDTH setting to disable 1135 a selected number of the originally enabled lanes such that the total number of enabled lanes at the endpoint component is no greater than RNCTRL.Maxwidth. As a result, the normal operation state is entered 1140 with the new bus width of RNCTRL.Maxwidth.

Referring to comparison step 1110, it is also necessary to negotiate increases in link width, which corresponds to RNCTRL.Maxwidth being greater than the current negotiated link width (N>M). The route depends whether the current negotiated link width is less than the maximum link width (K) (e.g., the maximum that would be negotiated without a RNCTR.Maxwidth constraint, which is obtained during the initial power-up negotiation process) and upon whether the upstream device is adapted to support renegotiation by entering the configuration state directly from the recovery state.

The condition N>M and M<K is evaluated 1145 to see if further increase in link width is possible. If no further increase in link width is permitted, a return 1160 is made to the operational state with the bandwidth unchanged (since it is already at a maximum). If the link width can be increased, a determination is made whether the upstream device supports 1150 a fast bus width re-negotiation protocol that enters 1155 the configuration state directly from the recovery state. If the upstream device does not support a fast bus width renegotiation protocol, a protocol is used 1170 which enters the configuration state through the loopback state.

Negotiating an increase in link width depends upon whether the upstream device (e.g., the root complex) obeys the standard PCI Express™ protocol or has been modified to support a fast renegotiation. This is due, in part, to the nature of the specific mandatory protocols of the PCI Express™ specification. For the case that the upstream device does not support fast renegotiation, the entry to the configuration step may require entry via the loopback step, as illustrated in FIG. 5. However, if the upstream device supports fast negotiation, the entry to the configuration may be made directly from the configuration step, as illustrated in FIG. 4.

FIG. 12 illustrates an embodiment in which fast renegotiation of an increase in bus width is not supported by the upstream device. A determination that the upstream devices does not support fast renegotiation can be made 1205, for example, by examining a vendor ID field to determine the functionality of the upstream device. As previously described, the endpoint device LTSSM enters 1210 the recovery state and sends out TSI/TS2 training sets according to the current link and lane numbers during the Recovery.RcvrLock and Recovery.RcvrCfg sub-states. Additionally, the loopback bit in the training control field of the TS1/TS2 training sets is asserted.

In response to the TS1/TS2 ordered sets, the upstream device is driven 1220 into the Recovery state. Consequently, both ends of the link are driven into the Recovery.Idle sub-state of the recovery state after Recovery.RcvrLock and Recovery.RcvrCfg.

At the Recovery.Idle sub-state, the endpoint device sends out 1230 TS1 ordered sets on all configured lanes with the loopback bit in the training control field asserted. The endpoint device LTSSM enters the Loopback.Entry sub-state of the loopback state as the loopback master.

The upstream component is driven 1240 into the Loopback.Active sub-state as a loopback slave after receiving TS1 with loopback asserted while it is in the Recovery.Idle state. The upstream component then starts looping back the TS1 ordered sets with the loopback bit asserted that are received from the endpoint component.

In response to the endpoint component receiving the TS1 looped back from the upstream device, it skips the Loopback.Active state and enters 1250 Loopback.Exit immediately. This drives the upstream device into the Loopback.Exit state as well.

The endpoint component can then exit 1260 the loopback state. In one embodiment, the endpoint component enters the Polling.Active sub-state with all K lanes that are maximally configured without a RNCTR.Maxwidth constraint. The (K–M) lanes, which are enabled at the initial power-up negotiation process but are disabled for power reduction, are re-activated here to start the link-width increase negotiation process.

The upstream component exits 1270 the Loopback.Exit sub-state and enters the Detect.Quiet sub-state. In response to the K active lanes in the endpoint component, the upstream component will be driven into the Polling state with K active lanes as well.

Note that by re-activating the disabled lanes in the endpoint component, the re-negotiation begins with the maximum number of active lanes negotiated during the initial power-up negotiation. For a maximum number of lanes, K, and a RNCTRL.Maxwidth value of N, the endpoint device selects the minimum of N and K as the new lane width, and use the information to constrain the re-negotiation process. During configuration, lanes that are selected to be disabled send 1280 pad symbols in the link and lane numbers fields of TS1/TS2 ordered sets. In this way, the bus exits 1290 to the normal operating state at the end of configuration with a new link width no greater than RNCTRL.Maxwidth.

FIGS. 13A and 13B are a flowchart illustrating a bus width upgrade protocol that is supported by the upstream device. A determination that the upstream device supports 1305 fast renegotiation may be made, for example, using a vendor ID field of a training set.

The endpoint device enters 1310 the Recovery.RcvrLock sub-state. The M currently configured lanes send out TS1 training sets according to the current link and lane numbers. The previously disabled (K–M) lanes among the K maximally negotiated lanes are re-activated. For the re-activated lanes, the PAD symbol is sent in the link number and lane number fields of TS1 training sets, and their receivers have to perform bit and symbol lock as well during the Recovery.RcvrCfg state. The next state, Recover.RcvrCfg, for the endpoint device is not reached until all of the K lanes achieve bit and symbol lock.

The upstream device is driven 1320 into the Recovery.RcvrLock state upon detecting TS1 ordered sets from the endpoint device on the M currently configured lanes. Once an electrical idle exit is detected on the formerly disabled (K–M) lanes, the lanes are re-activated and start sending TS1 ordered sets with link and lane number fields set to PAD. The next state Recover.RcvrCfg is not entered until all K lanes achieve bit and symbol lock.

At the Recovery.RcvrCfg sub-state, all of the M currently configured lanes send out 1330 and detect TS2 ordered sets. The (K–M) re-activated lanes also send TS1 or TS2 ordered sets in order to keep the bit and symbol lock on the other side of the link. Both ends of the link then enter the Recovery.Idle sub-state.

At the Recovery.Idle sub-state, the endpoint device sends out 1340 TS1 ordered sets with link and lane number fields set to PAD on all K enabled lanes (including M currently configured and (K–M) re-activated lanes). The endpoint device LTSSM enters the configuration sub-state Configuration.Linkwidth.Start.

After receiving a TS1 ordered set with the lane numbers set to PAD, the upstream component is driven 1350 into the Configuration state and begins the lane width re-negotiation with K active lanes.

Note that by re-activating the disabled lanes in both components, the re-negotiation begins with the maximum number of active lanes negotiated during the initial power-up negotiation. For a maximum number of lanes, K, and a RNCTRL.Maxwidth value of N, the endpoint selects the minimum of N and K as the new link width, and uses the information to constrain the re-negotiation process. During Configuration sub-states, lanes that are to be disabled send 1360 pad symbols in the link and lane numbers fields of TS1/TS2 ordered sets. In this way, the bus then exits 1370 to the normal operating state at the end of configuration with a new link width no greater than RNCTRL.Maxwidth.

One application of the present invention is in a computer system having a CPU, GPU, and software applications with different bus bandwidth needs. In particular, some high performance graphics applications (e.g., three-dimensional graphics applications) have extremely high bandwidth requirements in comparison to other applications, such as text processing or two-dimensional graphics. Additionally, a GPU executing a high performance graphics applications requires substantial computing power. In a graphics system the bus of the present invention preferably is part of a larger power saving architecture in which the GPU and the bus have power-saving modes that permit both GPU power and bus power to be decreased when high performance graphics applications are not in use.

FIG. 14 is a flow chart of one method of saving power in a graphics system. Responsive to detecting an idle state for the system, 3-D graphics may be disabled 1410. This may include, for example, switching a GPU to a low-power consumption state. Correspondingly, the bus is configured to have a minimum link width 1420 (e.g., one lane). In response to detecting exit 1430 from an idle state, the bus is configured 1440 to have a link width appropriate for the application. The GPU is configured 1450 for the need of the application.

Some of the benefits of the present invention will now be discussed with regards to an exemplary implementation. In a high performance graphics system, a bus having 16 active lanes may be required. For the operational (L0) state, the power consumption may be 100 milliwatts per active lane per direction, or 200 milliwatts per lane. Thus, in 16 lane configuration a PCI Express™ bus may consume 3.2 Watts of power. However, in the present invention, the link width may be negotiated to a low number for other applications, e.g., one lane for a text processing application. Negotiating a 1 lane configuration when high bus bandwidth is not required results in a savings of 3 Watts, or 94% of the bus power.

Additionally, the present invention permits the bus width to be negotiated in a comparatively short time. In a PCI Express implementation, completion of the detect and polling states takes at least 12 milliseconds. By comparison, the configuration state takes only a few microseconds to complete. Consequently, the present invention permits a comparatively fast renegotiation. In the context of a graphics system, the renegotiation process may be sufficiently rapid that it does not interfere with a user's experience.

An embodiment of the present invention relates to a computer storage product with a computer-readable medium having computer code thereon for performing various computer-implemented operations. The media and computer code may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts. Examples of computer-readable media include, but are not limited to: magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROMs and holographic devices; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and execute program code, such as application-specific integrated circuits (“ASICs”), programmable logic devices (“PLDs”) and ROM and RAM devices. Examples of computer code include machine code, such as produced by a compiler, and files containing higher-level code that are executed by a computer using an interpreter. For example, an embodiment of the invention may be implemented using Java, C++, or other object-oriented programming language and development tools. Another embodiment of the invention may be implemented in hardwired circuitry in place of, or in combination with, machine-executable software instructions.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention. 

1. A method of bus power management, comprising: for a first operational state having a first link bandwidth requirement for a bus, negotiating a first link width between a first bus interface and a second bus interface disposed at opposite ends of said bus based on said first link bandwidth requirement, said first link width having a first integer number, M, of active data lanes that is less than or equal to a maximum integer number, K, of operable data lanes; and in response to detecting a second operational state having a second link bandwidth requirement, negotiating a second link width between said first bus interface and said second bus interface based on said second link bandwidth requirement said second link width having a second integer number, N, of active data lanes, where N is not equal to M, and N is also less than or equal to K; wherein the number of active data lanes that is negotiated is based on a bandwidth constraint to reduce the power consumption associated with active data lanes.
 2. The method of claim 1, wherein said negotiating said first link width comprises: selecting M=K.
 3. The method of claim 2, wherein negotiating said second link width comprises: selecting N to be a minimum number sufficient to provide the bandwidth required for said second operational state.
 4. The method of claim 1, further comprising: prior to negotiating said first link width, detecting operable data lanes and establishing common communication parameters on said lanes.
 5. The method of claim 4, further comprising: storing initialization data describing operable data lanes and their communication parameters for subsequent negotiations.
 6. The method of claim 1, wherein said second operational state is associated with a change in a type of software application being executed.
 7. The method of claim 1, wherein said second operational state is associated with a change in a state of operation of a central processing unit.
 8. The method of claim 7, wherein said state of operation of said central processing unit is selected from the group consisting of a busy state, an idle state, and a run state.
 9. The method of claim 1, wherein said negotiating said second link width comprises: generating a disable signal to disable operable data lanes to reduce link bandwidth.
 10. The method of claim 1, wherein said negotiating said second link width comprises: enabling previously disabled operable data lanes to increase link bandwidth.
 11. The method of claim 1, wherein negotiating said second link width further comprises: a first component triggering entry of a second component into a recovery state; said first component further generating training sets during said recovery state to trigger said first component and said second component to enter a configuration state in which lane widths are negotiated for said bus.
 12. The method of claim 11, wherein said first component generates training sets in said configuration state that selectively disable a subset of operable data lanes.
 13. The method of claim 1, wherein negotiating said second link width further comprises: a first component triggering entry of a second component into a recovery state; said first component further generating training sets during said recovery state to initiate said second component to enter a loopback state; said first component generating training sets during loopback to initiate said second component to enter a detect state, from which a configuration state is entered in which lane width is negotiated; wherein re-negotiation of lane widths occurs without a reset phase.
 14. A method of bus power management: detecting operable data lanes in a detect state; establishing common communication parameters for said data lanes in a subsequent polling state; entering a configuration state; for a first operational state having a first link bandwidth requirement, negotiating a first link width between a first bus interface at one end of a bus and a second bus interface at another end of the bus, said first link width having a first integer number, M, of active data lanes that is less than or equal to a maximum integer number, K, of operable data lanes; entering an operational state with said first integer number, M, of active data lanes associated as a link between bus components; triggering entry back into said configuration state in response to detecting a second operational state having a second link bandwidth requirement; negotiating a second link width between said first bus interface and said second bus interface, said second link width having a second integer number, N, of active data lanes, where N is not equal to M, and N is also less than or equal to K; and entering said second operational state with said second integer number, N, of active data lanes associated as said link between said bus components wherein the number of active data lanes that is negotiated is based on a bandwidth constraint to reduce the power consumption associated with active data lanes.
 15. The method of claim 14, wherein triggering entry back into said configuration state comprises: in response to detecting said second operational state, entering a recovery state; and from said recovery state, entering said configuration state.
 16. The method of claim 14, wherein triggering entry back into said configuration state further comprises: in response to detecting said second operational state, entering a recovery state; and from said recovery state, entering a loopback state; from said loopback state, re-entering said detect state; wherein a reset state is avoided.
 17. A data bus, comprising: a first data interface disposed at one end of said data bus; a second data interface disposed at the other end of said data bus; a plurality of data lanes for communicating data between said first data interface and said second data interface; said first data interface and said second data interface adapted to initiate a negotiation for a number of active lanes corresponding to a subset of operable data lanes; said first data interface and said second data interface re-negotiating the number of active data lanes to place a constraint on the number of active data lanes associated to form a link in response to a re-negotiation signal; wherein the number of active data lanes that is negotiated is based on a bandwidth constraint to reduce the power consumption associated with active data lanes.
 18. The bus of claim 17, wherein said first and second interfaces are PCI Express™ compliant.
 19. The bus of claim 17, wherein one of said interfaces selectively disables a number of operable data lanes to adjust bandwidth.
 20. The bus of claim 17, wherein one of the interfaces selectively enables a number of operable data lanes to adjust bandwidth.
 21. A computing system, comprising: a root complex; a CPU coupled to the root complex for executing a software application stored in a memory; an endpoint device associating a graphical processing unit (GPU); a bus having a plurality of data lanes coupling the root complex to the endpoint device; the bus having bus interface disposed at two ends of the bus configured to associate active data lanes into a common link having a lane width corresponding to a number of active data lanes selected to adapt to the bandwidth requirements of the GPU; wherein the number of active data lanes that is negotiated is based on a bandwidth constraint to reduce the power consumption associated with active data lanes.
 22. The computing system of claim 21, wherein the bus includes bus interfaces disposed in the root complex and the endpoint device, wherein the interfaces are configured to re-negotiate a lane width of the bus in response to a re-negotiation signal.
 23. The computing system of claim 21, wherein the bus retains detect and polling information from an initial set-up procedure for use during subsequent lane width negotiations. 